The present invention relates generally to the construction of filter bags used in fabric-type dust and particulate collection, and more particularly, to an improved filter bag construction of hybrid construction.
Stringent environmental controls on industrial emissions virtually require many industries to remove particulate matter from dust-laden air and gas streams before the air and gas streams are exhausted into the atmosphere. This removal is typically done by filtration and involves passing the gas, or airstream, through a fabric filter. This filtration is typically accomplished in what is known as a "baghouse" or "dust collector" which is an enclosed area having what is known as a tubesheet with one or more openings formed therein that receive and support elongated filter "bags" that are made of a filter fabric.
The air, or gas stream, is forced into the baghouse where it passes through the filter bags, trapping the particulate matter in the air stream on the bag's surface so as to collect the particulate matter. The filtered air may then be exhausted to the atmosphere or used in another industrial application. After a predetermined time of operation, the particulate matter has accumulated on the filter bags and the bags need to be cleaned.
This cleaning may be accomplished by mechanically shaking the filter bags to dislodge the accumulated particulate matter. Cleaning may also be accomplished by reversing the flow of the air or gas stream which passes through the bag in order to dislodge the accumulated particulate matter. Cleaning may still further be effected by using a pulsed jet of air that is forced through the filter fabric. All of this type of cleaning, as well as the normal operation of the baghouse, subjects the filter fabric of the bags to repeated stretching which eventually may result in the deterioration of the filter bags. The filter bags must be checked periodically and manually replaced at intervals.
Filter bags used in baghouses may be classified into two different types of construction. In the first type of construction, the filter bag is formed from a filter fabric and is stretched over an internal metal cage that supports the bag within the tubesheet opening. A metal collar assembly is required to hold the filter bag and the support cage together and to provide the combined assembly with a flange that engages the tubesheet. U.S. Pat. No. 4,138,234, issued Feb. 6, 1979, describes such a filter bag construction.
As shown in this '234 patent, it can be seen that the metal wire support cage is position inside the filter bag and extends for the entire length. The wire support cage is attached to a collar that has an annular gasket supported thereon. A metal venturi may be also attached to the collar and extend part way into the bag to provide for a smooth transition of the air or gas stream when entering the bag. Certain problems are inherent with this type of filter bag construction. For example, the metal cages are prone to wear out. The metal support cage causes the overall bag assembly to have a rather heavy weight that renders its replacement and removal difficult and requires increased labor. Additionally, the metal support assembly is composed of multiple parts that must be assembled prior to insertion of the filter bag assembly into the baghouse. This assembly requires labor and these parts must also be checked during service and maintenance intervals. This increases the labor cost associated with assembling and servicing these filter bags.
Another type of filter bag assembly construction is described in U.S. Pat. No. 5,632,791, issued May 27, 1997. In this type of construction, a pleated filter bag is used and is supported by an internal plastic support cage. The internal support and the bag are attached to a pliable plastic cap, or annular header portion, that is formed of polyurethane or the like. The polyurethane header member may often be molded, or potted, directly to the filter bag. This header member is formed with a diameter that is slightly less than the diameter of the tubesheet opening. However, this filter bag construction is costly, because each such filter bag assembly has an individual mold for the header member having a specific diameter that matches the tubesheet opening size. This increases the overall cost of the filter bag assemblies in that the cost for each size mold must be considered. Additionally, when the header member is molded to the top end of the pleated filter media, an inadequate seal may result from the molding under elevated temperature operation that would create an opening between the two members through which air or gas could bypass the filter fabric. Still further, the molded header member may tear or rip during installation or removal and may lose strength due to high temperature conditions.
Still further, other filter bag constructions in the prior art utilize a filter bag with a molded header member in consideration with a metal snap band on the inside diameter of the header member. This snap band serves to secure the bag assembly to the tubesheet opening. This snap band must be removed when the filter bag assembly is installed and removed. This is an extra step that increases the labor cost of installation and removal.
The present invention is accordingly directed to an improved filter bag assembly of hybrid material construction that avoids the aforementioned shortcomings.